So, when the user clicks the “Continue with Facebook” button, the URL from the postMessage payload executes in the proxy iframe, which leads to the execution of the JavaScript on Facebook.com.

On the right side is "Continue." The Continue button is noticeably much bigger and brighter, and located where you'd usually go to complete a transaction.

This will cause a dialog box containing Continue and Go Back buttons to appear. Hit the Continue button, and the game will open. Wait briefly for the app to load all the required files and assets.